The invention relates to an endoprosthesis, in particular for a hip joint, having an anchoring component which can be anchored to the bone and which is connected to a joint component. Endprostheses are implanted when a satisfactory therapeutic effect can no longer be achieved by operations performed to preserve the joint. This is particularly the case where hip joints are concerned when advanced arthropathy, necrosis of the head of the femur or a medial fracture of the neck of the femur are present.
One disadvantage of all known endoprostheses is that, although these are initially immovably joined with the bone, they can become loose after a certain amount of time. Living bone is constantly undergoing changes which may vary considerably from location to location. If, in the course of such changes, individual sections of the bone should become weakened in the peripheral area of the prosthesis, the bone at this point will give way and the prosthesis will move. Even if such movements occur only in the micro-range, the original stability is no longer guaranteed. Under unstable conditions, the force flows and load distributions on the bone are constantly changing and the bone cannot start any reparative reactions because the conditions for oriented growth are lacking. Instead, at the points where excessive stress builds up (stress reaction) the bone will react by locally atrophying as it does at points where the stress is too low (stress protection). This leads to a progressive loosening of the prosthesis.
Such restructuring of the bone must be expected to occur even years after the prosthesis has successfully healed into place, because bone is a living organ and it reacts to changes in lifestyle, diet and other circumstances. Conventional endoprostheses do not take account of this situation.
A loosening process of this kind is one of the major problems encountered in orthopaedic surgery and it frequently necessitates replacement of the prosthesis. The corrective operations required in this case are, however, much more problem-ridden than the primary interventions because not only is the bone tissue which was removed for the primary implantation now missing, but also usually as a result of the interventions because not only is the bone tissue which was removed for the primary implantation now missing, but also usually as a result of the loosened implant or because of the resulting abrasion, additional serious defects are created which make it extremely difficult or indeed impossible to re-fix the implant.
In the case of hip endoprostheses sometimes only the head and neck of the femur are removed and replaced by a head prosthesis.
In the prior art head prostheses the anchoring component consists usually of a metal spike, for example of titanium, which is fixed in the medullary cavity of the femur and it is fitted with a laterally projecting pin whose free end bears a head section. In order to implant such a prosthesis it is necessary to remove the head and neck of the femur as well as most of the sponglous bone at the proximal end of the femur. The spike is then cemented into the femoral medullary cavity using polymethylmethacrylate, or it is driven into the bone without any cement; in this latter case it is primarily necessary to achieve optimal contact between the bone and the implant, and this may be reinforced by secondary growth of new bone tissue.
German Patent Application DE-A 28 45 231 describes a joint prosthesis in which a joint component is provided with a shaft which is attached to the bone by means of a tension bolt.
German Patent Application DE-A 28 54 334 describes an endoprosthesis of complicated design for a hip joint. The prosthesis consists of a shaft running longitudinally in the medullary cavity of the bone, and a neck part running in the direction of the neck of the femur and carrying the head of the joint. In this design, the shaft in the medullary cavity is rigidly connected to the femoral neck part.
German Patent Application DE-A 30 17 953 reveals an endoprosthesis for the head of a femur in which the head component is rigidly and non-slidingly connected via a threaded bolt with an anchoring component consisting of a sleeve inserted into the bone and of a trochanteral plate bolted to the outer surface of the bone. In this case the threaded bolt acts as a tension anchor by means of which the prosthesis is pretensioned against the bone.
German Patent Application DE-A 34 20 035 describes a joint prosthesis in which the head of the joint is rigidly attached to the bone via a projecting member.
One common feature of all these prior art joint prostheses is that they are non-flexibly attached to the bone in a manner which does not allow for later restructuring processes taking place in the bone.
When an acetabular prosthesis is implanted, still intact bone material is sometimes removed from the pelvic bone in order to be able to cement the socket into place or to anchor it in position without the use of cement. This known method of carrying out the operation, using known types of endoprosthesis, has a number of disadvantages. For example, large amounts of intact bone material have to be sacrificed, including in particular those parts of the bone in the proximal area of the femur which, because of their ideal trajectory, provide for optimum absorption of forces.
In the area of the femoral shaft, the existing force flows are reoriented. Under natural conditions, mainly pure bending stresses, harmoniously distributed from top to bottom, predominate in this area, but when the prosthesis has been implanted forces are generated which run mainly from inside the bone to the outside and these are combined with shear forces and a relatively abrupt transition from low stress in the portions of the bone in contact with the prosthesis to extreme stress at the lower tip of the anchoring spike. The bone itself is forced to react to the changed conditions by undergoing restructuring. This applies not only to the femoral but also to the acetabular conditions. Restructuring always involves the simultaneous loss of existing bone and the growth of new bone. If the amount of bone lost exceeds the amount of new bone growth, the prosthesis will lose its grip in certain sections and it will start to come loose.
Also, when the prior art hip joint prostheses are used, insufficient attention is paid to the individually different conditions in the patient's anatomy. For example, the angle between the shaft and the neck of the femur ("CCD angle") varies in size from person to person (the physiological range is taken to be 115.degree. to 140.degree. degrees). Furthermore, the neck of the femur does not run in the sagittal plane of the body but is tilted forward and towards this plane at an angle of varying size ("AT angle", physiological range approx. 10.degree. to 40.degree. degrees). The sizes of both angles are proportionately interdependent. Prior art endoprostheses do not take these facts into account and only one standard dimension is used for all hips. As a result, this almost always leads to functionally incorrect placing of the joint and consequently also a change in the way in which forces are introduced into the shaft of the femur. To the extent that they are capable, bones and soft tissues must adapt to the new conditions. Pain, restricted mobility and premature loosening of the prosthesis are all possible consequences. Custom-made prostheses are not immediately available, they are for the most part inaccurate, and they cost up to ten times as much. Furthermore, computer tomography, which exposes the patient to large amounts of radiation, is needed to determine the anatomical situation prior to performing the operation. In this case, too, the functionally incorrect siting of the joint involves the risk that the implanted prosthesis will come loose, with all the disadvantages already mentioned above.
Colonization of the surface of the prosthesis by bacteria is a not infrequent and much feared complication following implantation. Such contamination usually makes it necessary to remove the prosthesis completely. However, removing an endoprosthesis which has become firmly interlocked with the bone growing around it, or removing the bone cement, is a very difficult task which frequently results in severe additional damage to the surrounding bone structures. Replacing the prosthesis with a new one cannot be done at all or only after the bone has been allowed to heal for several years. If an infection exists in the area of a femoral head prosthesis of prior art design, then because the prosthesis is anchored in the medullary cavity of the femur it must be expected that the infection will spread as far as the knee joint.
Anchoring the head component of the prosthesis in the shaft of the bone is not only a non-physiological approach but also it is not always technically simple to accomplish. Usually, an extensive set of additional surgical instruments is needed to prepare the bone in the exact manner required. Furthermore, opening up the medullary cavity always results in heavy and persistent bleeding which necessitates the administration of multiple units of stored blood.